1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile, and a developing device used therefor.
2. Description of the Related Art
Vertical agitation type developing devices horizontally arranged in parallel are used as conventional developing devices for reducing the size of a full color image forming apparatus. In such a vertical agitation type developing device as illustrated in FIG. 8, a function separation method is adopted in which a developing chamber 41a and an agitating chamber 41b are vertically disposed, and toner is supplied to a developing sleeve 44 from the developing chamber 41a and is collected in the agitating chamber 41b. Therefore, toner density nonuniformity and image density nonuniformity resulting therefrom can be prevented, and thus a uniform image having no lengthwise density difference can be obtained.
In the vertical agitation type developing device 4 adopting the function separation method, if the fluidity of developer is varied as a result of a control operation such as T/D control for durability or charging amount adjustment, circulation of the developer may lean to one side in a developing container 41, and the developer may easily stagnate on a scooping portion. Here, T/D denotes a weight ratio of toner relative to developer.
The conveying of developer is very sensitively affected by the fluidity of the developer because the developer is squeezed by a second conveying screw 41e while being scooped up from the lower agitating chamber 41b to the upper developing chamber 41a. If developer stagnation occurs, developer rotates together with the developing sleeve 44 after a developing operation instead of being stripped from the developing sleeve 44 at the downstream side of the agitating chamber 41b (corotation phenomenon). If the level of developer stagnation increases, the developing device 4 may overflow with the developer.
A common method used as a countermeasure against such overflow is to form a groove having a depth of several tens micrometers (μm) in the developing sleeve 44 for easily conveying developer. FIG. 9 is a front view illustrating a developing device having a grooved sleeve in the related art. Referring to FIG. 9, a groove is formed at regular intervals around the surface of a developing sleeve 44 of the related art along a longitudinal rotational axis thereof. Furthermore, in a blast method also used as a countermeasure against overflow of developer, glass beads are injected to the surface of the developing sleeve 44 to roughen the surface of the developing sleeve 44. In those cases, since the developing sleeve 44 can convey developer forcibly, it is possible to reduce the amount of developer stagnating on a stripping pole between repelling like poles of a magnet disposed in the developing sleeve 44.
In addition, as a method of stabilizing developer distribution in a vertical agitation type developing device, a return screw is disposed between an agitating screw and a developing sleeve (Japanese Patent Laid-Open Nos. 6-51634 and 11-84874). In Japanese Patent Laid-Open No. 6-51634, when developer is collected to an agitating chamber from a developing region by rotating a return screw and an agitating screw in opposite directions, the amount of developer is large in the vicinity of an upper scooping portion, thereby preventing stripped developer from sticking back to a developing sleeve.
However, in the structure where a groove is formed in a developing sleeve, a corotation phenomenon may occur in which developer is not stripped from the developing sleeve but is rotated together with the developing sleeve after a developing operation.
Further, in the configuration disclosed in Japanese Patent Laid-Open No. 6-51634, all the developer splattered upward by the return screw may not reach the agitating screw but some may return to the developing sleeve. If it becomes difficult to convey developer as the fluidity of the developer is lowered by aging, the scooping portion may scoop developer insufficiently, the return screw may convey the developer insufficiently, and the developer may be collected from the developing sleeve insufficiently, thereby causing a corotation phenomenon.
Furthermore, the configuration disclosed in Japanese Patent Laid-Open No. 11-84874 does not sufficiently cope with the change of the distribution of developer according to the variation of fluidity caused by durability deterioration, T/D variation, and greenhouse environment.